In an electronic circuit, heat radiated from a heat producing component such as a power transistor is usually dissipated by a heat sink mounted on the component. However, mounting such a heat sink is a costly process and therefore a heat dissipation structure having the heat sink cannot be reduced in cost. Furthermore, the heat sink cannot be used for dissipating heat produced by the surface mount device.
To solve the problems, a case for housing an electronic component and a circuit substrate is used as a heat sink and the substrate is thermally coupled with the case via a heat dissipation member. Heat dissipation structures including grease as a heat dissipation member are proposed in JP-A-2002-83912,JP-A-2002-93962,JP-A-2002-217346, and JP-A-5-335452. The grease is easily applied to electronic components because of its flexibility and the thermal coupling between a case and the electronic component or a substrate is appropriately maintained. Moreover, thermal stress is not applied to the electronic component because the grease is not cured.
However, it is difficult to retain the grease in a predetermined position due to its flow ability when vibrations occur. The heat dissipation ability of the grease is degraded with time. When the circuit substrate is installed in a vehicle in which temperatures greatly vary, the grease tends to move in the direction of gravitational force during cooling cycles.
In JP-A-2002-93962, a hole is provided in the heat sink for holding the grease when the grease is squeezed out due to a decrease in volume of grease containing space. The volume of grease containing space is decreased when the substrate or the heat sink is warped as a result of cooling cycles. The movement of the grease is reduced by the hole. However, the grease tends to spread over the contact surface of the heat sink faster than to flow into the hole or well. Therefore, the movement of the grease is not properly reduced.
In JP-A-2002-217346 and JP-A-5-335452A, a well is provided in the heat sink or the substrate for holding the grease. The grease is placed in the well so that the movement of the grease over the contact surface is reduced. However, the well becomes an obstacle to installation of the electronic component on the substrate and therefore flexibility in circuit design is reduced. Furthermore, a larger mounting area is required for the substrate.